1. Field of the Invention
The present invention relates to a storage medium having a game program stored thereon and a game apparatus, and more particularly to a storage medium having stored thereon a game program and a game apparatus for displaying a game image using a game field of a curved surface.
2. Description of the Background Art
Conventionally, widespread is a game apparatus for providing a game which allows a player to control a player object displayed on a game screen. For example, as disclosed in Japanese Laid-Open Patent Publication No. 2002-939 (hereinafter, referred to as Patent Document 1) and Japanese Laid-Open Patent Publication No. 10-198819 (hereinafter, referred to as Patent Document 2), the player object and other objects positioned on a plane game field in a virtual space are taken by a virtual camera and represented in an image, thereby generating a game screen.
As shown in FIGS. 21 to 23 of Patent Document 1, a character (central character) and an opponent are positioned on a plane game field and represented in an image so as to position the character at the center of a game screen. Further, as shown in FIGS. 4 to 9 of Patent Document 2, a water surface is represented in an image using a plane field (polygon) and the image is outputted to a monitor.
Here, when a cylindrical or a spherical game field is used, a virtual space can be represented in a game image with increased stereoscopic effect as compared to a game image generated using the plane game field. However, when the game field has a cylindrical or spherical surface, objects and the like which move on the game field must be moved in accordance with a shape of the surface. Therefore, a processing load for moving the object along the field of a curved surface is increased as compared to a process of moving the object on the plane field, thereby complicating a calculation for moving the object.
For example, in the case of the plane game field, a direction of gravity acting on the game field in the virtual space is perpendicular to a flat plane, and the direction of gravity is constant with respect to the virtual space. On the other hand, in the case of the cylindrical or the spherical game field, the direction of gravity is a radial direction of the game field, that is, the direction of gravity is a direction radiating into the virtual space, whereby a process, which has to be performed in consideration of the direction of gravity, is complicated. Moreover, when the object is moved along the cylindrical or the spherical game field, the object is moved in a curved direction, thereby making the calculation for moving the object difficult.
Further, when an object in the virtual space represented on a screen is designated using a pointing device such as a touch panel for designating an input position or a coordinate point on the screen, complicated is a process for determining whether or not the object disposed in the cylindrical or the spherical game field is designated. Specifically, an image model representing a player object and respective image models representing other objects are disposed on the cylindrical or spherical game field, and the state of the cylindrical or spherical game field is taken by a virtual camera and represented in an image, thereby generating a game screen. That is, each of the image models is disposed in a direction (direction of gravity) perpendicular to a position of each of the image models in the game field, whereby each of the image models is disposed in a different reference direction based on its direction of gravity. When an image model designated by a player with the pointing device is identified in the virtual space, a processing load is increased for accurate determination because the image models have complicated shapes and the game field having the image models disposed therein are cylindrical or spherical.